Problem: 3 crayons cost $3.75. Which equation would help determine the cost of 7 crayons?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. We can write the fact that 3 crayons cost $3.75 as a proportion: $\dfrac{3}{\$3.75}$ Let $x$ represent the unknown cost of 7 crayons. Since 7 crayons cost $x$ , we have the following proportion: $\dfrac{7}{x}$ The cost changes along with the number of crayons purchased, and so the two proportions are equivalent. $\dfrac{3}{\$3.75} = \dfrac{7}{x}$